New research shows a significant drop in U.S. breast cancer cases in recent years, but only in white and Hispanic women possibly because they gave up hormone replacement therapy in greater numbers than minority groups.

In April 2002, a large trial suggested that the combination of estrogen and progestin used to treat menopause symptoms raised the risk of breast cancer and heart disease. Therefore, many women stopped using hormone replacement therapy, which resulted in a sharp decreasing of breast cancer rates during 2002 and 2003. However, this decline was confined to Caucasians, the researchers reported.

“In women in the age group from 50 to 69 – those more likely to use hormone-replacement therapy – we found that the reduction in invasive breast cancer with estrogen-receptor positive tumors was like 13 percent for whites, 11 percent for Hispanics, about 4 percent for Asian of Pacific Islanders and no change for African-Americans,” Dezheng Huo, an epidemiologist at the University of Chicago Medical Center and lead author of the study, said as quoted by the Washington Post.

Huo and his colleagues based their study on data from 17 cancer registries, covering about 26 percent of the U.S. population.

The researchers blamed the differences in biology for much of the racial disparity. “We suspect that the widespread discontinuation of menopausal hormone use had a greater effect on Caucasians,” Huo said. Black women have less estrogen-receptor positive cancers, and so the decline in HRT use might not have such a dramatic effect on their cancer, he speculated.

Another reason that might be responsible for the racial differences could be that black women may have used HRT less frequently than white women, and so again would not be as affected by the declining use of hormones.

Almost 80 percent of breast cancers in Caucasians are estrogen receptor positive, a type of tumor that depends on estrogen to grow, said Huo, compared to about 60 percent in African Americans. That proportion drops to about 30 percent among Nigerian women.

The research was supported by Breast Cancer SPORE grant from the National Cancer Institute.